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20F07 — Acorn BBC Micro Logo on Famico

6502 Assembly is the language used to program the Famicom, BBC Micro and Commodore 64 computers.

This page is a collection of notes on the basics of 6502 assembly, assembled from various guides and tutorial. Prior to learning assembly, I would recommend building a basic understanding of binary numbers.

Lexicon

Directives are commands you send to the assembler to do things like locating code in memory. They start with . and are indented. This sample directive tells the assembler to put the code starting at memory location $8000, which is inside the game ROM area. Labels are aligned to the far left and have a : at the end. The label is just something you use to organize your code and make it easier to read. The assembler translates the label into an address.

Opcodes are the instructions that the processor will run, and are indented like the directives. In this sample, JMP is the opcode that tells the processor to jump to the MyFunction label. Operands are additional information for the opcode. Opcodes have between one and three operands. In this example the #$FF is the operand:

Comments are to help you understand in English what the code is doing. When you write code and come back later, the comments will save you. You do not need a comment on every line, but should have enough to explain what is happening. Comments start with a ; and are completely ignored by the assembler. They can be put anywhere horizontally, but are usually spaced beyond the long lines.

  .org $8000
MyFunction:                    ; A comment
  LDA #$FF
  JMP MyFunction

Styleguide

Major comments are prefixed with two semi-colons, and minor comments are found at the end of a line on the 32nd column if available. Variables and subroutines are lowercase, constants and vectors are uppercase, and routines are capitalized.

;; Variables

  .enum $0000                  ; Zero Page variables
pos_x                   .dsb 1
pos_y                   .dsb 1
  .ende

;; Constants

SPRITE_Y            .equ $0200
SPRITE_X            .equ $0203

RESET:
  NOP

Forever:
  JMP Forever

NMI:
  RTI

;; Routines

Check_Collision: 
  LDA pos_y
  CMP #$88                     ; Floor is at 32y
  BCC @done
  LDA #$88
  STA pos_y
@done:
  RTS

;; Tables

Table_Name:
  .db $40,$46,$4c,$52,$58,$5e,$63,$68

;; Vectors

  .pad $FFFA
  .dw NMI
  .dw RESET
  .dw 0
  .incbin "src/sprite.chr"

The lin6 linter is used to enfore this style on the various assembly projects found on this site.

Registers

The 6502 handles data in its registers, each of which holds one byte(8-bits) of data. There are a total of three general use and two special purpose registers:

Note: When you use X it adds the value of X to the memory address and uses the 16-bit value at that address to do the write. Whereas when you use Y it adds the value of Y to the address stored in the memory address it's reading from instead. 6502 is little-endian, so $0200 is stored as $00 $02 in memory.

Addressing

The 6502 has 9 major(13 in total) addressing modes, or ways of accessing memory.

Immediate#aaThe value given is a number to be used immediately by the instruction. For example, LDA #$99 loads the value $99 into the accumulator.
AbsoluteaaaaThe value given is the address (16-bits) of a memory location that contains the 8-bit value to be used. For example, STA $3E32 stores the present value of the accumulator in memory location $3E32.
Zero PageaaThe first 256 memory locations ($0000-00FF) are called "zero page". The next 256 instructions ($0100-01FF) are page 1, etc. Instructions making use of the zero page save memory by not using an extra $00 to indicate the high part of the address.
ImpliedMany instructions are only one byte in length and do not reference memory. These are said to be using implied addressing. For example, CLC, DEX & TYA.
Indirect Absolute(aaaa)Only used by JMP (JuMP). It takes the given address and uses it as a pointer to the low part of a 16-bit address in memory, then jumps to that address. For example, JMP ($2345) or, jump to the address in $2345 low and $2346 high
Absolute Indexed,X/Yaaaa,XThe final address is found by taking the given address as a base and adding the current value of the X or Y register to it as an offset. So, LDA $F453,X where X contains 3 Load the accumulator with the contents of address $F453 + 3 = $F456.
Zero Page Indexed,X/Yaa,XSame as Absolute Indexed but the given address is in the zero page thereby saving a byte of memory.
Indexed Indirect(aa,X)Find the 16-bit address starting at the given location plus the current X register. The value is the contents of that address. For example, LDA ($B4,X) where X contains 6 gives an address of $B4 + 6 = $BA. If $BA and $BB contain $12 and $EE respectively, then the final address is $EE12. The value at location $EE12 is put in the accumulator.
Indirect Indexed(aa),YFind the 16-bit address contained in the given location ( and the one following). Add to that address the contents of the Y register. Fetch the value stored at that address. For example, LDA ($B4),Y where Y contains 6 If $B4 contains $EE and $B5 contains $12 then the value at memory location $12EE + Y (6) = $12F4 is fetched and put in the accumulator.

Common Opcodes

Load/Store opcodes
LDA #$0ALoaD the value 0A into the accumulator A. The number part of the opcode can be a value or an address. If the value is zero, the zero flag will be set.
LDX $0000LoaD the value at address $0000 into the index register X. If the value is zero, the zero flag will be set.
LDY #$FF LoaD the value $FF into the index register Y. If the value is zero, the zero flag will be set.
STA $2000STore the value from accumulator A into the address $2000. The number part must be an address.
STX $4016STore value in X into $4016. The number part must be an address.
STY $0101STore Y into $0101. The number part must be an address.
TAX Transfer the value from A into X. If the value is zero, the zero flag will be set.
TAY Transfer A into Y. If the value is zero, the zero flag will be set.
TXA Transfer X into A. If the value is zero, the zero flag will be set.
TYA Transfer Y into A. If the value is zero, the zero flag will be set.
Math opcodes
ADC #$01 ADd with Carry. A = A + $01 + carry. If the result is zero, the zero flag will be set
SBC #$80 SuBtract with Carry. A = A - $80 - (1 - carry). If the result is zero, the zero flag will be set
CLC CLear Carry flag in status register. Usually this should be done before ADC
SEC SEt Carry flag in status register. Usually this should be done before SBC
INC $0100INCrement value at address $0100. If the result is zero, the zero flag will be set
DEC $0001DECrement $0001. If the result is zero, the zero flag will be set
INY INcrement Y register. If the result is zero, the zero flag will be set
INX INcrement X register. If the result is zero, the zero flag will be set
DEY DEcrement Y. If the result is zero, the zero flag will be set
DEX DEcrement X. If the result is zero, the zero flag will be set
ASL A Arithmetic Shift Left. Shift all bits one position to the left. This is a multiply by 2. If the result is zero, the zero flag will be set
LSR $6000Logical Shift Right. Shift all bits one position to the right. This is a divide by 2. If the result is zero, the zero flag will be set
Comparison opcodes
CMP #$01 CoMPare A to the value $01. This actually does a subtract, but does not keep the result. Instead you check the status register to check for equal, . Less than, or greater than
CPX $0050ComPare X to the value at address $0050
CPY #$FF ComPare Y to the value $FF
Control-Flow opcodes
JMP $8000JuMP to $8000, continue running code there
BEQ $FF00Branch if EQual, contnue running code there. First you would do a CMP, which clears or sets the zero flag. Then the BEQ will check the zero flag. If zero is set (values were equal) the code jumps to $FF00 and runs there. If zero is clear (values not equal) there is no jump, runs next instruction
BNE $FF00Branch if Not Equal - opposite above, jump is made when zero flag is clear

Compare

The compare instructions set or clear three of the status flags (Carry, Zero, and Negative) that can be tested with branch instructions, without altering the contents of the operand. There are three types of compare instructions:

InstructionDescription
CMPCompare Memory and Accumulator
CPXCompare Memory and IndexX
CPYCompare Memory and Index Y

The CMP instruction supports eight different addressing modes, the same ones supported by the ADC and SBC instructions. Since the X and Y registers function primarily as counters and indexes, the CPX and CPY instructions do not require this elaborate addressing capability and operate with just three addressing modes (immediate, absolute, and zero page).

The compare instructions subtract (without carry) an immediate value or the contents of a memory location from the addressed register, but do not save the result in the register. The only indications of the results are the states of the three status flags: Negative (N), Zero (Z), and Carry (C). The combination of these three flags indicate whether the register contents are less than, equal to (the same as), or greater than the operand "data" (the immediate value or contents of the addressed memory location. The table below summarizes the result indicators for the compare instructions.

Compare Result NZC
A, X, or Y < Memory *00
A, X, or Y = Memory 011
A, X, or Y > Memory *01

The compare instructions serve only one purpose; they provide information that can be tested by a subsequent branch instruction. For example, to branch if the contents of a register are less than an immediate or memory value, you would follow the compare instruction with a Branch on Carry Clear (BCC) instruction, as shown by the following:

Comparing Memory to the Accumulator

  CMP  $20    ; Accumulator less than location $20?
  BCC  THERE
HERE:
  ; No, continue execution here.
THERE:
  ; Execute this if Accumulator is less than location $20.

Use of Branch Instructions with Compare

To Branch IfFollow compare instruction with
For unsigned numbersFor signed numbers
Register is less than dataBCC THEREBMI THERE
Register is equal to dataBEQ THEREBEQ THERE
Register is greater than dataBEQ HERE
BCS THERE
BEQ HERE
BPL THERE
Register is less than or equal to dataBCC THERE
BEQ THERE
BMI THERE
BEQ THERE
Register is greater than or equal to dataBCS THEREBPL THERE

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